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Windmill Terms


I was unable to find a glossary of windmill terms and so decided to create one.  At this point there will be many more terms that I have yet to add, but I am doing it as I come across them.


To get a basic understanding of the main parts take a look at the page on How a smock mill works , this has a list of terms, not in alphabetical order but explaining it in the order from the sails inwards.


A Frame - Is the supporting frame that folds up the bottom of the main drive shaft, effectively transferring the weight out to supports.

Auxiliary drive - Other drives coming off that can drive other equipment.

Bedstone  - The Bedstone is the bottom of a pair of millstones. It does not move. The upper stone is called the Runner Stone.

Brake Wheel  - The Brake Wheel is the main driving wheel in a Smock or Tower mill, and in some post mills. It is carried on the Windshaft and drives the Wallower on the Upright Shaft. Connected to the windshaft this is clamped by blocks or a brake to stop the mill or slow it down. It is a large cog wheel that meshes with the wallover.

Bridge tree - An adjustable, horizontal beam, supporting the vertical stone spindle which allows the gap between the grinding stones to be varied.

Buck  - A term for the body of a post-mill.

Crown Tree - The Crown Tree is the central, single baulk of timber, usually oak, that rests on top of the post in a post mill. Attached to it are the side-girts and the rest of the frame of the buck.

Centrifugal governor - As a weight spins around it's thrown outwards, and this is relative to the speed. This action is connected to a mechanism that can make adjustments to the sweeps so that the windmill runs  constantly at the same speed, as the wind goes up and down. The whole system has a lot of weight and therefore inertia so tends to average out variations in the wind.

Curb, truck wheels and rollers - The mechanism that allows the cap to rotate on top of the main body.

Drive shaft or main drive shaft - This is the vertical shaft that rotates and off which all other drives come to run any part of the equipment.

Fantail - Sitting on the cap, or with a post mill on an arm behind, this detects changes in wind direction and via a drive arrangement is able to turn the cap so that the windmill always faces into the wind.

Great Spur Wheel  - The Great Spur Wheel is carried on the Upright Shaft. It drives the Stone Nuts. Millstones driven by the Great Spur Wheel can be either Overdrift or Underdrift. A large gearwheel which, together with a smaller gear called a pinion, connects two parallel shafts and, in a corn mill, drives the stone nuts.

Head Wheel  - The Head Wheel is carried on the Windshaft in a Post Mill and has a brake around its circumference. It drives a Stone Nut. Millstones driven by the Head Wheel are always Overdrift stones.

Iron Cross - The fitting that the stocks connect into.

Overdrift - Millstones driven from above are known as Overdrift stones.

Pintle  - The pivot centering a post mill on top of the main post.

Runner Stone  - The Runner Stone is the topmost of a pair of millstones. It is driven by the Stone Nut. The lower stone is called a Bedstone.

Sack hoist - A mechanical hoist that is driven from an arrangement from the wallower that allows sacks to be lifted vertically right up through the mill. Often you will see a continuous chain arrangement where the chain goes through a hole in or between two doors that open up as sacks come up and shut after so that no one can fall down through the hole.

Sails - The Sails are the source of power in a windmill. They are carried on the Windshaft. Most windmills had four sails, although some had five (Boston), six (Waltham, Lincs) or eight sails (Heckington, Lincs) and there is one recorded twelve sailed windmill (Cottenham, Cambs).

Common Sails have a lattice framework over which a sailcloth is spread. These were the earliest type of sails in northern European windmills.
Spring Sails, invented in 1772 by Andrew Meikle, have shutters adjusted by a spring. Each sail is adjusted individually and, as with Common Sails the mill has to be stopped to enable an adjustment to be made.
Roller Reefing Sails, invented in 1789 by Stephen Hooper, use a canvas strip wound around a roller in the place of shutters. The mill does not have to be stopped in order to adjust these sails.
Patent Sails, invented in 1819 by William Cubitt, combine the shutters of the Spring Sail with the automatic adjustment of the Roller Reefing Sail. Single Patents have shutters on the trailing side of the sail, Double Patents have shutters on both sides of the sail for its whole length.

Samson Head  - An iron collar and plate bearing that fits over the pintle of a post-mill's post, that supports the weight of the crown tree, around which the buck of the mill is constructed. An example is visible at High Salvington Windmill, in West Sussex.

Stone assembly and stones - used for grinding.

Stone Nut  - The Stone Nut is a small gear driven by the Great Spur Wheel, Head Wheel, or Tail Wheel. It drives the Runner Stone either from above (Overdrift) or below (Underdrift). A Pinion which engages with the spur wheel and drives the millstones in a corn mill.

Stock - Is the supporting arms onto which the sales are fitted, when you see a windmill with just a number of large poles instead of sails its the stocks that you can see.

Sweep - This is the sail.

Tail Wheel  - The Tail Wheel is carried on the Windshaft in a Post Mill and drives a Stone Nut. Millstones driven by the Tail Wheel are always Overdrift stones.

Tentering screw and brayer - Adjusts the gap between the millstones. This is the mechanism by which the miller may adjust the gap between his millstones while they are running. As he turns the giant wing-nut he raises or lowers the end of the cross beam, the centre of which in turn supports one end of the bridge beam that carries the shaft with the runner-stone on top. Having set the stones correctly, at the start of a day's work, the only adjustment he needs is to compensate for the expansion of the stones as they warm up during use. This tweaking may well require less than one full revolution of the tentering screw throughout the day.

Trestle - The Trestle is the substructure of a Post Mill, usually enclosed in a protective structure called a roundhouse, which also serves as a storage facility. Post mills without a roundhouse are called Open Trestle Post Mills.

Underdrift - Millstones driven from beneath are known as Underdrift stones.

Upright Shaft - The Upright Shaft is the main vertical shaft found in Smock and Tower mills. It is also found in some Post mills. It carries the Wallower at its top end, and a Great Spur Wheel at the bottom end. The Great Spur Wheel drives two or more Stone nuts.

Wallower  - The Wallower is a driven gear at the top of the Upright Shaft in Smock, Tower and some Post mills. It is driven by the Brake Wheel. A cog wheel that is meshing with the brakewheel, that transfers the drive from the windshaft coming in from the side to the drive shaft that runs vertically. In effect its a very large cog wheel at the top of the drive shaft.

Windshaft  -The Windshaft carries the Sails and also the Brake Wheel (Smock and Tower mills, and in some Post mills) or the Head Wheel and Tail Wheel in a Post Mill. Windshafts can be wholly made of wood, or wood with a cast iron Poll End (where the Sails are mounted) or entirely of cast iron. The shaft that the iron cross is connected to, and sticks out of the cap.


More Terms

A very large English Heritage thesaurus of all terms used in all types of older structures can be reached by clicking here for structural items, and clicking here for machinery items. You will find most windmill related items are in the structural items


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